Disconnects are required when the equipment is too far from, or not within sight of the breaker (or branch circuit disconnect) (commonly known as "Serviceman disconnects" or "Serviceman switches"). This is required so that while you're working on the equipment, it's less likely somebody will accidentally energize the equipment (flip the breaker on).
National Electrical Code 2014
Chapter 4 Equipment for General Use
Article 440 Air-conditioning and Refrigerating Equipment
440.14 Location. Disconnecting means shall be located within sight from and readily accessible from the air-conditioning or refrigerating equipment. The disconnecting means shall be permitted to be installed on or within the air-conditioning or refrigerating equipment.
The disconnecting means shall not be located on panels that are designed to allow access to the air-conditioning or refrigeration equipment or to obscure the equipment nameplate(s).
Fuses are used for overload protection, mostly for motors. The fuse is typically sized to the motor load it's protecting, and may or may not be a different size than the circuit breaker. If a motor can't spin, it will quickly overload, overheat, and burn up. A fuse is used to prevent that.
For more information on protecting the equipment, and sizing the protection. See the rest of article 440. As Mjohns answer points out, the manufacturer's documentation rules, so make sure you read and understand it.
ecmag.com's interpretation of 314.16(B)(1) Conductor Fill:
There is an exception under the conductor-fill provision that pertains to luminaire (fixture) conductors. An equipment-grounding conductor or four or fewer luminaire (fixture) wires smaller than 14 AWG, or both, entering a box from a domed luminaire (fixture) or similar canopy and terminating within that box, can be omitted from box-fill calculations. Under certain conditions, up to four luminaire (fixture) conductors and one equipment-grounding conductor can be installed but not counted in the box-fill calculation. The conductors must be 16 AWG or smaller and must enter the box from a domed luminaire (fixture) or similar canopy.
I always wondered why the wires on lights are so flimsy; that's why, so you don't have to count them in box fills.
Best Answer
The following is from the 1999 NEC (my 1990 NEC hard copy has the same wording in the same location) so it has been there a long long time:
Good luck with your project!